A-a gradient at sea level
This computerized calculation assumes that the current barometric pressure is 760 mm Hg and that the patient is close to sea level.
Modify the FiO2 if the patient is not on room air. There is no satisfactory method for accurately estimating the FiO2 for patients managed with nasal canulas.
The formula for doing this calculation by hand is (Calculated O2 - Measured O2) or (PAO2 - PaO2)
The PAO2 may be calculated with the following formula:
PAO2 = FiO2 × (760-47) - (PaCO2/0.8)